Three-dimensional optical imaging of biological cells and tissues has become increasingly critical for biomedical studies and medical diagnosis. Although images in the lateral direction (i.e., parallel to the sample surface) can be routinely acquires in parallel (e.g., using light-sheet microscopy) or at high speed (e.g., using high-speed scanning microscopy), no current imaging modality can image an axial slice (i.e., perpendicular to the sample surface) in parallel, limiting our ability to visualize fast biological processes happening across different depths. In this proposed study, we aim to overcome this limitation by developing a new technology capable of imaging axial slices of a sample in parallel. The key concept is the proposed use of an array of 45?- tilted high-aspect-ratio micro mirrors arranged along the axial direction, which can demultiplex image signals emitted from different depth positions by exploiting their different wavefronts. Each micro mirror has a dimension comparable to the diffraction limited beam width. Consequently it behaves effectively as a confocal pinhole or slit, providing optical sectioning capability. Building upon our preliminary study, in the proposed program we will first design the proposed high-aspect-ratio micro mirror array and fabricate it by using micromolding. We will then develop the proposed axial slice light-sheet imaging system by incorporating the high-aspect-ratio micro mirror array device. The prototype system will be characterized and optimized. High- resolution axial slice light sheet imaging of fluorescent microspheres and 3D spheroid tumor cell culture complex systems will be demonstrated.